Structures built on earth foundations cause compression and settlement of the underlying soil. Such compression and settlements may continue for a long time after such a foundation is loaded by the overlying weight of the structure. The total magnitude of settlement, and the time duration of any significant settlement, are usually very important to the proper design, construction, and performance of most important engineered structures.
It is sometimes possible to improve the load carrying characteristics of a foundation site by pre-loading the site with a height of added soil that is equivilent in weight to the proposed structure for the site, commonly referred to as surcharging the site. In such a case it is necessary for the foundation design engineer to monitor the time-rate results of settlement beneath the surcharge in order to compare actual with predicted results, and so finally to verify that all significant compressions to be expected under the proposed permanent structure have already taken place.
In order to monitor settlements beneath such surcharges, it has been common practise to place flat steel plates on the existing ground surface at numerous points around the site, and to extend pipes, connected at the bottom to the plates, up through the surcharge, as it is placed by heavy, earth-moving equipment. By keeping track of the precise length of pipe added above the plates, and by having survey crews establish the elevation of the upper, exposed end of the pipes from time to time during construction of the surcharge and thereafter, the elevation of the buried plate, and hence the time-rate settlement of the ground surface may be determined.
Some of the shortcomings of such a system have been the risk of damage and loss to the settlement plate riser pipe by the earth-moving equipment, the increased costs of placing the surcharge due to interference of the pipes with free movement of the equipment, the costs for the two or three man survey crew necessary to add additional pipe lengths as the surcharge to the top of each pipe to determine the settlement of the plate, the limited number of settlement plates that can be accomodated on the average job, and the numerous occasions for accidental data errors.
The advantages of the instant invention over the prior method is that it may be installed in shallow trenches in the existing ground surface before surcharge construction begins; it may be operated thereafter for settlement observations with no interference whatsoever to the surcharge construction; it may be operated by one man, with no attention to the system necessary in the interval of time between sets of observations; it provides a more comprehensive evaluation of the complete settlement profile beneath the surcharge than is possible with earlier methods which monitored a very limited number of discreet points within the site.
Another ideal application of the instant invention is in the observation of compressions and settlements both beneath and within earth-fill dams, used to impound water in reservoirs. Such earth-fill dams are commonly constructed of several zones of soil of different properties. Commonly at least one zone comprises the principal seepage barrier to the water impounded on the upstream side of the dam, while outer, coarser zones of soil or rock provide the necessary strength against rupture of the embankment, due to pressure of the water, or other disturbing forces such as earthquakes. Usually, a free-draining zone is found to be necessary near the downstream face of the seepage barrier zone in order to remove the small amount of seepage that is unavoidable, and to conduct it safely and quickly from the dam before it can saturate and weaken the downstream zone(s) responsible for maintaining stability only.
Foundation settlements that are severe in total magnitude, and particularly where they vary abruptly across the base of the dam, that is, severe differential settlement, may cause cracking or shearing of the overlying, heavily compacted earth-fill zones comprising the dam. Such cracking can lead to failure of the dam, through excessive seepage past the seepage barrier, offseting and blinding of sections in the free-draining zones, and weakening of the stability zones.
The instant invention may be employed in the foundation of the dam before construction begins, and again at several higher levels in the embankment above. The only time the system must be protected from other construction activities is during the actual installation period of one to several days, depending on the total length of system desired at any level. Thereafter, the system is operated without interference to dam construction, and the compressions and settlements in the foundation and within the embankment are monitored to determine whether they are in the expected tolerances for the dam. Various remedial measures are available to the engineer in case he believes the tolerances are being exceeded, including such steps as slowing the rate of construction, altering grading and moisture content of the various zones, and altering dimensions of the zones.
The instant invention is also convenient for installation beneath, or within a large, interconnected complex of buildings, such as in heavy industrial plants, nuclear and fossil fueled power generating plants, and levees and canal superstructures. Such buildings usually afford some degree of access to the usual survey crews, operating with geodetic levels and rods, to monitor visible points on the outside walls. However, here again such access may be objectionable and limited, and may be more costly than use of the instant invention.